When laying a foundation of a tower construction such as a wind turbine, a reinforced concrete element is often cast and subsequently, the turbine tower is positioned on the concrete element and fixed hereto. A simple way of fixing the tower to the concrete element is to anchor substantially vertical anchor rods in the concrete element, and attach these to a flange being attached to the tower wall.
U.S. Pat. No. 5,586,417 discloses an example of such a tower construction. The foundation is formed within a ground pit and externally and internally back filled. The lower end of the foundation has a circumferential ring fully embedded therein and sets of inner and outer circumferentially spaced bolts have their lower ends anchored to the anchor ring, their upper ends projecting up outwardly of the top of the foundation and a majority of the midportions thereof free of connection with the cementitious material of which the foundation is constructed. The base flange of a tubular tower is positioned downwardly upon the upper end of the foundation with the upper ends of the inner and outer sets of bolts projecting upwardly through holes provided therefore in the base flange and nuts are threaded downwardly upon the upper ends of the bolts and against the base flange. The nuts are highly torqued in order to place the bolts in heavy tension and to thus place substantially the entire length of the cylindrical foundation in heavy axial compression.
Due to the heavy weight of the tower construction, the anchor rods and the concrete element will be exposed to a large pressure at points carrying the tower. The anchor rods tend to bend as a consequence hereof, though being kept approximately in position by the concrete element. When the turbine tower is in position, bolts attached to the anchor rods are tightened to provide tension on the anchor rods, thereby post-stressing the concrete element in great compression and leaving the anchor rods in static tension. Even though the foundation is leveled before positioning the turbine tower, a large pressure is put on the anchor rods when positioning the tower.